Asymmetric nuclear-resonance broadening, as originating, e.g., from magnetic-hyperfine-field distributions in magnetic alloys, has strong effects on the time evolution of nuclear forward scattering of synchrotron radiation. In thin samples of an Invar alloy, resonance broadening and the resulting dephasing in time cause a fast decay of the coherent scattering signal. In thick samples, the asymmetry of the broadening strongly affects dynamical scattering. Quantum beat and dynamical beat blend into a fast hybrid beat with thickness dependent period and field distribution sensitive modulation.

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In 1954 Dicke predicted the accelerated initial decay of multiple atomic excitations 1 , laying the foundation for the concept of superradiance. Further studies 2-4 suggested that emission of the total energy was similarly accelerated, provided that the system reaches the inversion threshold. Superradiant emission of the total energy has been confirmed by numerous studies [4][5][6][7][8][9][10][11][12] , yet the acceleration of the initial decay has not been experimentally demonstrated. Here we use resonant diffraction of X-rays from the Mössbauer transition 13 of 57 Fe nuclei to investigate superradiant decay, photon by photon, along the entire chain of the de-excitation cascade of up to 68 simultaneous coherent nuclear excitations created by a pulse of an X-ray free-electron laser. We find agreement with Dicke's theory 1 for the accelerated initial decay as the number of excitations is increased. We also find that our results are in agreement with a simple statistical model, providing a necessary baseline for discussing further properties of superradiance, within and beyond the low-excitation regime. Dicke's model introduces superradiance as an accelerated initial decay of multiple atomic excitations, and provides exact predictions for the ensemble behaviour as a function of the number of atoms and number of excitations in the system

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